It measures about sixteen and a half metres long, fourteen metres wide, and rises to roughly a metre in height, and it is composed almost entirely of burnt and fire-cracked stone. That material is the whole point. This is a fulacht fia, one of the most common prehistoric monument types in Ireland, and yet one that most people walk past without a second thought, mistaking it for an unremarkable rise in the ground.

A fulacht fia is, in essence, the debris left behind by a Bronze Age cooking method. The typical arrangement involved a timber trough sunk into the ground near a water source, filled with water, and then heated by dropping stones that had been fired in an open hearth directly into it. The stones, repeatedly heated and quenched, eventually shattered and became useless for further heating. Over generations of use, the discarded fragments accumulated into the horseshoe-shaped or kidney-shaped mounds that survive across the Irish landscape today, sometimes in their thousands. At Coolacullig, the opening of that horseshoe, some fourteen metres wide, faces west, which may reflect the position of the original trough or hearth in relation to the mound's growth. Whether these sites were purely for cooking, or also served other purposes such as textile processing or bathing, remains a matter of ongoing discussion among archaeologists, but the burnt stone evidence is consistent wherever they appear.